The present invention relates to a method of operating a radio communication system having a random access channel. The present invention further relates to such a system and to primary and secondary stations for use in such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that such techniques are equally applicable to use in other mobile radio systems.
A random access channel is a normal component of a radio communication system, enabling a Mobile Station (MS) to send short messages to a Base Station (BS). Applications include signalling to the BS when the MS is turned on, sending a packet of data to the BS when the MS is not engaged in a call, and requesting the BS to allocate a resource for the MS to use.
A variety of methods have been proposed for the implementation of a random access channel. One example, for use in UMTS (in a wide band Code Division Multiple Access (CDMA) frequency division duplex mode), is shown in FIG. 2. A MS accesses an uplink channel by sending a preamble (P) 102 followed by a message packet (MSG) 104. A BS can start to process the message packet 104 when it has received the preamble 102. Once the BS has received the message 104 it sends an acknowledgement (ACK) 106 on a downlink channel to indicate to the MS that the message 104 has been received and decoded correctly. Typically the preamble 102 might last 1 ms, followed by a 0.25 ms time interval then a 10 ms message 104.
A problem with this method is the lack of any power control. Ideally the MS transmit power should be chosen so that the preamble 102 and message 104 are received by the BS at the power level required for correct decoding of the message 104. If the MS transmits at too high a power level its signal may swamp other signals received at the BS, while if it transmits at too low a power level its signal may not be received at all by the BS.
Here the MS determines the power at which to transmit by measuring the power received from the BS over the downlink channel and using this measurement to estimate the path loss in the uplink channel. However, such a method is not very reliable. Two problems are that the required received power at the BS is not constant but varies with radio channel conditions and speed of the MS and the uplink and downlink channels are not necessarily reciprocal.
If the message 104 is not received correctly by the BS, no acknowledgement 106 is transmitted. The MS determines from the lack of an acknowledgement 106 that its access attempt has failed and, after waiting for a back-off period, repeats the attempt. This scheme has the disadvantage that several retransmissions may be needed, giving rise to the possibility of significant delays.
An improved scheme is shown in FIG. 3. The MS first transmits a preamble 102 at a reduced power level. If the BS receives and decodes the preamble correctly it transmits a preamble acknowledgement (A) 204. In the example shown in FIG. 3, after the first preamble is transmitted no acknowledgement is returned in the slot 202 allocated for it (which might typically be 1ms in length). The MS therefore transmits another preamble 102 at a higher power level. Again no acknowledgement is received in the slot 202, so the MS transmits another preamble 102 at a still higher power. This is received and decoded by the BS, which transmits an acknowledgement 204. Having received this, the MS is able to transmit the message 104.
However, this improved scheme still does not provide closed loop power control for the message 104, which means that it will require a higher Eb/No (energy per bit/noise density) than normal data transmissions. Hence the MS needs to use more transmitter power on average, generating more interference than usual, and system uplink capacity may be wasted.
An object of the present invention is to provide closed loop power control for a random access channel.
According to a first aspect of the present invention there is provided a method of operating a radio communication system having a random access channel for enabling a secondary station to transmit a message to a primary station, comprising the secondary station transmitting a preamble encoded with a signature on the random access channel to the primary station and subsequently transmitting the message, characterised by the primary station transmitting a control channel including power control information after successful reception of the preamble, in response to which the secondary station adjusts the output power of its transmitter.
According to a second aspect of the present invention there is provided a radio communication system comprising a primary station, a secondary station and a random access channel for transmission of messages from the secondary station to the primary station, the secondary station having means for transmitting a preamble encoded with a signature on the random access channel, characterised in that the primary station has means for transmitting a control channel including power control information after successful reception of the preamble, and the secondary station has means for adjusting the output power of its transmitter in response to the reception of the control channel.
According to a third aspect of the present invention there is provided a primary station for use in a radio communication system having a random access channel for the transmission of messages from a secondary station to the primary station, the primary station having means for reception of a preamble encoded with a signature on the random access channel transmitted by the secondary station and means for determining the power of a transmission received from the secondary, characterised in that means are provided for transmitting a control channel after successful reception of the preamble, the control channel including power control information for the secondary station to alter the output power of its transmitter.
According to a fourth aspect of the present invention there is provided a secondary station comprising for use in a radio communication system having a random access channel for the transmission of messages to a primary station, the secondary station having means for transmitting a preamble encoded with a signature on the random access channel, characterised in that the secondary station has means for adjusting the output power of its transmitter in response to the reception of a control channel transmitted by the primary station.